This invention relates to the field of optical communication systems and, more specifically, to the extinction ratios of transmitters.
Extinction ratio is an important performance characteristic of laser transmitters used in optical communications systems. It is a measure of the amplitude of the digital modulation on the optical carrier and, therefore, affects the power penalty, or distance over which an optical fiber telecommunications system can reliably transmit and receive a signal. Standards for optical telecommunications systems, such as SONET, SDH, and Fibre Channel, specify minimum extinction ratio requirements for laser transmitters. Since extinction ratio is explicitly specified in these standards, it is important that any given laser transmitter, when its performance is measured on different test systems, yields a similar minimum extinction ratio value.
Another leading concern in optical communication systems is that the extinction ratio of a transmitter can be deteriorated by several factors, including temperature and amplifier response. If the extinction ratio becomes too small, an increase in noise results results in undesirable bit errors at an optical receiver detecting an output of the transmitter.
The present invention advantageously provides a method and apparatus for modifying the extinction ratio of a modulated optical signal by adapting a modulator driver signal in response to power level differences in spectral regions of the modulated signal.
In one embodiment of the invention, a method includes adapting an optical modulator driver signal to reduce a difference between a carrier frequency power level and a corresponding sideband power level.
In another embodiment of the invention, an optical modulation apparatus includes a modulator, for modulating an optical signal in response to a driver signal to produce a modulated optical signal having associated with it a carrier frequency power level and at least one sideband power level, and a controller, for adapting the driver signal in a manner tending to reduce differences between the carrier frequency and at least one sideband power level.